The present invention relates quantum dots, and more particularly to the use of quantum dots for security applications.
Quantum dots, including their optical and physical properties and methods of manufacture, are well known and described in the following publications:
1. Warren C. W. Chan, Shuming Nie, xe2x80x9cQuantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detectionxe2x80x9d, Science 281 (5385):2016
2. Marcel Burchez Jr., Mario Moronne, Peter Gin, Shimon Weiss, A. Paul Alivisatos, xe2x80x9cSemiconductor nanocrystals as Fluorescent Biological Labelsxe2x80x9d, 281 (5385):2013
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14. Warren C. W. Chan and Shuming Nie, xe2x80x9cQuantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detectionxe2x80x9d Science 1998 September 25;281:2016-2018.
The above publications describe methods for making quantum dots such as nanometer-sized crystals of CdSexe2x80x94CdS and ZnS-capped CdSe. The publications also describe physical and optical properties of these quantum dots. In particular, the publication by Chan et al (Publication 1) and in the publication by Burchez Jr., et al (Publication 2), describe quantum dots having the following fluorescence properties:
high fluorescence intensity, comparable to 20 molecules of Rhodamine 6G;
emission spectra one-third as wide as a typical organic dye-tagged latex sphere;
100 times lower photobleaching rate than typical organic dyes;
a long fluorescence lifetime, on the order of hundreds of nanoseconds.
fluorescence spectrum peak closely correlated to quantum dot diameter
As described at 2016-18 in the above-cited treatise by Chan and Nie, molecular conjugates of luminescent quantum dots are expected to offer substantial advantages over organic dyes. The properties of quantum dots result from quantum-size confinement, which occurs when metal and semiconductor particles are smaller than their exciton Bohr radii (about 1 to 5 nm) (2-4). Recent advances have resulted in the large-scale preparation of relatively monodisperse quantum dots (5-7), the characterization of their lattice structures (4), and the fabrication of quantum dots arrays (8-12) and light-emitting diodes (13, 14). For example, CdSe quantum dots passivated with a ZnS layer are strongly luminescent (35 to 50% quantum yield) at room temperature, and their emission wavelength can be tuned from the blue to the red wavelengths by changing the particle size (7, 15).
According to one aspect of this invention, quantum dots are used as fluorescent taggants in security inks, papers, plastics, explosives, or any other item or substance in which it is desired to provide a distinct signature or marking. Quantum dots are superior to standard fluorophores in this application because of their controllable fluorescence peak color, their characteristic narrow fluorescence spectra, their characteristically long fluorescence lifetimes, and the ability to make their fluorescence properties essentially independent of their immediate environment. Quantum dots of specific sizes, compositions and structures may be used to produce specific fluorescence, mixtures of quantum dots can be used to produce random patterns of spectrally varying fluorescence, and particular quantum dot structures can be used to provide desirable physical and optical properties.